Issue - Calls DROPS when Rebound in extension is enable with "SIP Trunk Re-Invite" enabled

[bravad]

It seems to have an engineering error on the part of 3CX.​

When an internal call is transferred to a mobile with "Rebound" feature active,
The caller is interrupted (drop call) when the call is accepted by the called party (after press "1" DTMF for accept call).
The called party remains connected but no one is connected with him, it is complete silence.

You can see in 3CX that there is a call connected but you cannot see the details in active calls.

How to reproduce the problem ?
1. Activate the rebound function in your extension for calls forwarded to your mobile
2. From another Extension, call the Extension that has the Rebound Active (feature).
3. Wait , when the called party has accepted the call by pressing 1, the caller call will drop but the called party remains connected.

The problem is easy to reproduce and recurs with any SIP trunk provider.
If the solution you offer me is to disable Re-Invite.. it's not a "fix", it's a workaround..

We use Re-Invite to let the provider manage "the calls hold" which allows the provider to control music on hold.
That works well. With the exception of the "Rebounds" calls which... don't work , Calls drops.

I attach 2 PCAPs. ( In ZIP File )
1 PCAP without Re-Invite, it works.
the other PCAP with Re-Invite, there is a problem

 

[YiannisH_3CX]

Hi @bravad

What is happening from a technical point of view is correct. Once you enable Re-invites on the trunks the PBX will send a Re-invite to transfer the call as its doing internally. By enabling Re-invites of the trunks you are telling the PBX that your provider supports this behavior.

From the captures it looks like that your provider does not support Invite messages without SDP and its rejecting the Invite. Invites without SDP are supported by the RFC and are used mostly in transfer scenarios like this one. The SDP will be sent in the ACK message from 3CX once the 200 OK message is received. So 3CX does not do anything wrong here.

You expect the PBX to send Re-invites for hold music but not for transfers. Unfortunately it does not work like that, if you enable the option you need to make sure that your provider supports all scenarios.

For the record we replicated the issue with a supported scenario that does support Re-invites with SDP and the transfer worked fine.

So you have 2 options here, you either disable Re-invites on your trunk or you find a provider that supports both functionalities.
On supported providers we leave the option disabled so the PBX can handle these scenarios properly.